The use of coupling capacitor voltage transformers (CCVT) with protective relays is well known in transmission line protection. They are used as part of the voltage step-down system to decrease the voltage from the level on the transmission line (approximately 132.8 kv) phase-phase to a 66.4 volt level which is used by the protective relay in its monitoring of voltage conditions on the line. Briefly, the capacitor portion of the CCVT accomplishes an initial step-down voltage function, typically to a range of 5 kv to 15 kv, while the transformer portion of the CCVT accomplishes the additional step-down function to 115 volts phase-phase or 66.4 volts phase-ground.
In the past, CCVTs have provided satisfactory results when used with conventional electromechanical relays. However, with the introduction and now relatively widespread use of high-speed, solid-state and numerical-based relays, the primary disadvantage of many CCVTs, i.e. a relatively poor transient response, is significantly accentuated. The poor transient response of a CCVT, in which the output of the CCVT (the secondary of the transformer portion thereof) does not follow the input voltage, is due to the energy storage elements (the capacitors and the inductor) in the CCVT, which require time to dissipate their stored energy.
When a fault occurs, the input voltage to the CCVT from the transmission line can drop dramatically to a relatively low voltage. The output of the CCVT, however, instead of replicating the ratioed input voltage (the ideal CCVT secondary output) accurately, produces a transient response (an output which does not exactly match the ideal secondary output). After a certain amount of time, the CCVT output again is coincident with the ratioed input voltage.
A high-speed protective relay, such as currently available solid-state relays, can actually respond to such transients. If the fault on the transmission line is outside of the defined zone 1 reach of the relay (the typical protective relay will have several zones of protection), the particular distance elements in the relay responsible for zone 1 protection can overreach in response to the transient and produce an undesirable output. One response to the overreach problem is to reduce the zone 1 reach, although the CCVT transient in some situations is sufficiently large or of sufficient duration that the reduction of zone 1 results in zone 1 protection being no longer effective. Because zone 1 elements are primary protection for the transmission line, it is very desirable to have as much of the transmission line protected by zone 1 elements as possible. Accordingly, it is undesirable to significantly reduce the zone 1 reach and thereby reduce that portion of the line protected by zone 1 elements.
The CCVT transient reduces the fundamental component of fault voltage, which in turn results in a reduction of the calculated fault impedance. Further, if the output of a CCVT is significantly distorted due to the presence of a transient, the distance relay is incapable of measuring the correct faulted line impedance during the time of transient distortion. The result of the CCVT transient is the overreaching by the distance elements, with the zone 1 element overreach having the most undesirable result, as these elements are usually instantaneous.
Hence, while a CCVT has certain advantages (low cost at high voltage installations) which make it useful with protective relays, its usefulness is diminished by the fast response of the new solid-state and microprocessor protective relays, to the point where any results achieved during the transient time are unreliable.